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United States Patent |
5,524,557
|
Spokoyny
|
June 11, 1996
|
Catalytic sulfur trioxide flue gas conditioning
Abstract
A method and apparatus for the selective control of the sulfur trioxide
concentration in flue gases, to enhance the ash removal efficiency of
electrostatic precipitators, which includes supporting a catalyst in the
path of the flue gas, catalytically converting a portion of the sulfur
dioxide contained within such flue gas to sulfur trioxide, by passing at
portion of such flue gases over a catalyst which is positioned within the
path; selectively varying the amount of sulfur trioxide produced by the
catalytically converting, by selectively changing the quantity of flue gas
flowing through such catalyst; and simultaneously maintaining the ratio of
the pressure loss coefficient of the flue gas flowing through the other
portion of such duct, in the vicinity of such catalyst, within a
predetermined range.
Inventors:
|
Spokoyny; Felix E. (Costa Mesa, CA)
|
Assignee:
|
Wahlco, Inc. (Santa Ana, CA)
|
Appl. No.:
|
349566 |
Filed:
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December 5, 1994 |
Current U.S. Class: |
110/203; 110/210; 110/211; 110/216; 110/345 |
Intern'l Class: |
F23J 015/00 |
Field of Search: |
110/203,210,211,216,345
|
References Cited
U.S. Patent Documents
4213403 | Jul., 1980 | Gomori | 110/216.
|
4599952 | Jul., 1986 | Meier | 110/216.
|
4898105 | Feb., 1990 | Rappolst et al. | 110/216.
|
4961908 | Oct., 1990 | Pennington et al. | 110/203.
|
4986196 | Jan., 1991 | Butch | 110/210.
|
5237939 | Aug., 1993 | Spokoyny et al. | 110/345.
|
5320052 | Jun., 1994 | Spokoyny et al. | 110/216.
|
Primary Examiner: Kwon; John T.
Attorney, Agent or Firm: Sandler; Howard E.
Claims
I claim:
1. A sulfur trioxide conditioning system for use in a fossil fuel-burning
facility having a main duct for transporting sulfur dioxide containing
flue gas from a boiler, through a heat recovery apparatus, and to
particulate removal equipment for subsequent discharge through a stack,
the sulfur trioxide conditioning system comprising:
a conditioning system support adapted to be disposed across at least a
major portion of the transverse cross-section of the main duct;
transversely spaced sections of catalytic converter means carried by said
support and operative in a manner to oxidize at least a portion of the
sulfur dioxide in the flue gas passing thereby to sulfur trioxide, said
sections creating a flow resistance with a first pressure loss coefficient
as to flue gases passing therethrough;
transversely spaced sections of flue gas flow resistance means carried by
said support, intermediate and adjacent said sections of flue gas
converter means, said sections of flue gas resistance means creating a
flow resistance with a second pressure loss coefficient as to flue gases
passing therethrough;
adjustment means operative to selectively adjust the quantity of flue gas
flow through said catalytic converter means, while simultaneously
adjusting the quantity of flue gas flow through said resistance means, in
a manner that when the quantity of flue gas flow through said converter
means is reduced, the flow through said resistance means is increased and,
conversely, when the flow through said converter means is increased, the
flow through said resistance means is increased; and
the ratio of the second pressure loss coefficient to the first pressure
loss coefficient being in the range of 0.2 to 2.
2. A sulfur trioxide conditioning system as specified in claim 1 wherein
said ratio is in the range of 0.5 to 1.0.
3. A sulfur trioxide conditioning system as specified in claim 1 wherein
the system is adapted to be operatively positioned in the main duct,
intermediate the boiler and the heat recovery apparatus.
4. A sulfur trioxide conditioning system as specified in claim 1 wherein
said catalytic converter means is active in the conversion of sulfur
dioxide to sulfur trioxide, at a temperature substantially no less than
five hundred (500) .degree.F.
5. A sulfur trioxide conditioning system as specified in claim 1 wherein
said catalytic converter means is active in the conversion of sulfur
dioxide to sulfur trioxide, at a temperature substantially no less than
six hundred and fifty (650) .degree.F.
6. A sulfur trioxide conditioning system as specified in claim 1 wherein
the catalytically active portion of said catalytic converter means is of a
precious metal catalyst.
7. A sulfur trioxide conditioning system as specified in claim 1 wherein
said transverse spacing between said sections of catalytic converter means
is uniform.
8. A sulfur trioxide conditioning system as specified in claim 7 wherein
said transverse spacing between said sections of flow resistance means is
uniform.
9. A sulfur trioxide conditioning system as specified in claim 1 wherein
said transverse spacing between said sections of catalytic converter means
is preselected.
10. A sulfur trioxide conditioning system as specified in claim 9 wherein
said transverse spacing between said sections of flow resistance means is
preselected.
11. A sulfur trioxide conditioning system as specified in claim 1 wherein
said adjustment means is movable transversely of such flue gas flow.
12. A sulfur trioxide conditioning system as specified in claim 11 wherein
said adjustment means is operative to selectively adjust the quantity of
flue gas flow through said catalytic converter means, by selectively
varying the face area of said sections of catalytic converter means,
through which such flue gas flows.
13. A sulfur trioxide conditioning system as specified in claim 1 wherein
said adjustment means incorporates a soot blowing assembly therewithin,
which is selectively operative to assist in dislodging accumulated
particulate from said catalytic converter means.
14. A sulfur trioxide conditioning system as specified in claim 13 wherein
said soot blowing assembly is selectively operative to assist in
dislodging accumulated particulate from said flow resistance means.
15. A sulfur trioxide conditioning system as specified in claim 11 wherein
an area of 10% to 50% of said system support (as measured transversely to
the flue gas flow) contains said sections of catalytic converter means
therewithin, and the balance of such area contains said sections of flow
resistance means therewithin.
16. A sulfur trioxide conditioning system as specified in claim 15 wherein
said adjustment means is configured and operative to selectively expose
and cover, with respect to the flue gas flowing through such conditioning
system, up to and including 100% of said sections of catalytic converter
means.
17. A sulfur trioxide conditioning system as specified in claim 16 wherein
said adjustment means includes a plurality of transversely spaced
elongated shield assemblies, upstream of said sections, with: the
transverse spacing of said shield assemblies being substantially equal to
the transverse spacing of said sections of catalytic converter means; the
axial elongation of said shield assemblies being substantially identical
to the extent of the elongation of said sections of catalytic converter
means; and the width of said shield assemblies being substantially
identical to the width of said sections of catalytic converter means.
18. A sulfur trioxide conditioning system as specified in claim 17 wherein
said shield assembly includes an upstream deflector section, which is
aerodynamically configured with respect to the flue gas stream passing
thereby.
19. A sulfur trioxide system as specified in claim 18 additionally
including downstream exit deflectors, in spaced registry, with respect to
the direction of flow of such flue gas stream, from said first upstream
deflector sections, said exit deflector sections being aerodynamically
configured with respect to the flue gas stream passing thereby.
20. A sulfur trioxide assembly as specified in claim 19 wherein said shield
assembly is operative in a manner that said upstream and exit deflector
sections are selectively movable, in tandem, transversely with respect to
the direction of the flue stream passing thereby.
21. A sulfur trioxide conditioning system as specified in claim 11 wherein
said adjustment means includes upstream deflector sections, which are
aerodynamically configured with respect to the flue gas stream passing
thereby.
22. A sulfur trioxide system as specified in claim 18 additionally
including downstream exit deflectors, in spaced registry, with respect to
the direction of flow of such flue gas stream, from said first upstream
deflector sections, said exit deflector sections being aerodynamically
configured with respect to the flue gas stream passing thereby.
23. A sulfur trioxide assembly as specified in claim 22 wherein said
adjustment means is operative in a manner that said upstream and exit
deflector sections are selectively movable, in tandem, transversely with
respect to the direction of the flue stream passing thereby.
24. A sulfur trioxide conditioning system as specified in claim 16 wherein
said adjustment means is operative to selectively expose and cover both
sides, with respect to the direction of movement of said adjustment means,
of said sections of catalytic converter means.
25. A sulfur trioxide conditioning system as specified in claim 1 wherein
the percentage of sulfur dioxide in the flue gas passing by such system,
which is converted to sulfur trioxide, is in the range or 0% to 20%, in
response to the operation of such adjustment means.
26. A sulfur trioxide conditioning system as specified in claim 1 wherein
the percentage of sulfur dioxide in the flue gas passing by such system,
which is converted to sulfur dioxide, is in the range or 0% to 3%, in
response to the operation of such adjustment means.
27. A sulfur trioxide conditioning system as specified in claim 1 wherein
said catalytic converter means comprises a plurality of catalyzed plates
configured in a manner to define flue gas flow paths therebetween, with
said plates being positioned so that flue gas flow may pass through said
flow paths.
28. A sulfur trioxide conditioning system as specified in claim 1 wherein
said catalytic converter means comprises a plurality of longitudinally
extending, transversely spaced honey-combed members, with said honey-comb
members being positioned so that the flue gas flows longitudinally
therethrough.
29. A sulfur trioxide conditioning system as specified in claim 1 which,
when positioned within such a duct, results in an additional pressure drop
of less than 5 inches water gage.
30. A sulfur trioxide conditioning system as specified in claim 1 which,
when positioned within such a duct, results in an additional pressure drop
of less than 2 inches water gage.
31. A method for treating the flue gas emitted from a coal burning boiler,
prior to flue gas passing to an electrostatic precipitator, comprising the
steps of:
passing such flue gas through a primary duct extending between the exit of
the boiler and the entrance to the precipitator;
catalytically converting a portion of the sulfur dioxide contained within
such flue gas to sulfur trioxide, by passing at portion of such flue gas
stream over a catalyst which is positioned within a portion of such duct;
selectively varying the amount of sulfur trioxide produced by said
catalytically converting, by selectively changing the quantity of flue gas
flowing through such catalyst; and
simultaneously maintaining the ratio of the pressure loss coefficient of
the flue gas flowing through the other portion of such duct, in the
vicinity of such catalyst, to the pressure loss coefficient of the flue
gas flowing through said catalyst, in the range of from 0.2 to 2.
32. A method as specified in claim 31 wherein said range is from 0.5 to
1.0.
33. A method as specified in claim 31 wherein said selectively varying is
by varying the face area of the catalyst through which such flue gas
flows.
Description
BACKGROUND OF THE INVENTION
This invention relates to power plant operations, and, more particularly,
to an approach for removing particulate matter from a flue gas stream
produced in a fossil fuel power plant, especially a coal-fired power
plant.
In a fossil fuel power plant, a fuel is burned in air to produce a flue
gas. The flue gas heats water in a boiler to generate steam, which turns a
turbine to produce power. After passing through various apparatus, the
flue gas is exhausted through a stack to the atmosphere.
The flue gas of certain fossil fuels (i.e. coal) includes solid particulate
matter and a variety of gaseous contaminants. The maximum permissible
emission levels of the particulate matter and gaseous contaminants are set
by laws and regulations. The maximum emission levels are typically far
less than the amounts present in the flue gas as it is produced, and
various types of gas treatment apparatus are usually provided to reduce
the particulate matter and gaseous contaminants in the flue gas before it
leaves the stack.
In many power plants, particulate matter in the gas stream is removed by
electrostatic precipitation. An electrostatic charge is applied to the
particulate matter in the flue gas, and the flue gas passes between
charged electrodes. The particulate matter is deposited upon the electrode
having the opposite charge to that of the particulate and is later
removed.
In plants burning coal, the fuel typically contains from about 0.2 percent
to about 6 percent sulfur, which at least in part oxidizes to sulfur
dioxide during combustion. A small part of the sulfur dioxide further
oxidizes to sulfur trioxide. Since the combustion air and the fuel also
contain moisture, the flue gas contains water vapor. The sulfur trioxide
and water vapor in the flue gas react to produce sulfuric acid, which
deposits upon the particulate matter. The sulfuric acid deposited upon the
particulate matter imparts a degree of electrical conductivity to the
particulate and promotes the electrostatic precipitation process.
If the fossil fuel contains too little sulfur, so that there is a
deficiency of sulfur trioxide, and thence sulfuric acid in the flue gas,
the electrostatic precipitator may not function properly because of the
high electrical resistivity of the particulate. It is therefore known to
add sulfur trioxide from an external source to the flue gas produced frown
burning low-sulfur fossil fuels. See, for example, U.S. Pat. No.
3,993,429.
In the '429 sulfur trioxide conditioning system, sulfur is burned to form
sulfur dioxide, which is passed over a catalyst to achieve further
oxidation to sulfur trioxide. The sulfur trioxide is injected into the
flue gas flow upstream of the electrostatic precipitator. The amount of
injected sulfur trioxide is controlled by varying the amount of sulfur
that is burned. Other similar sulfur trioxide systems, which have been
successfully used commercially, include a system which starts with a
sulfur dioxide feedstock, which is vaporized and then catalytically
converted to sulfur trioxide.
Sulfur trioxide injection systems, such as illustrated in the '429 patent,
work well and are widely used. In some instances, however, there are
drawbacks: high equipment capital costs; a constant supply of sulfur or
sulfur dioxide feedstock is required, and this feedstock must be safely
handled; the several components of the burning, catalyzing, and injecting
system must be kept in good working order; there is a substantial power
consumption associated with the process; when the plant or system goes
into stand-by condition, the system, at least from the converter forward,
must be purged to prevent excessive corrosion of the system and/or
blockage of the probe nozzles; the injection arrangement must be operative
over a range of boiler operating conditions in a manner that appropriate
mixing is achieved prior to the flue gas stream entering the precipitator;
because the conversion of the newly produced SO.sub.2 to SO.sub.s is not
always 100% efficient, trace amounts of excess SO.sub.2 may be produced;
in many instances, significant runs of hot gas insulated duct-work must be
included, together with complicated and costly manifold assemblies; and
the like.
U.S. Pat. No. 5,011,516 describes an alternate approach to the types of
systems illustrated in the '429 patent, and teaches an arrangement wherein
a slip stream of flue gas is drawn from the main flow and passed over a
catalyst. A portion of the sulfur dioxide in the slip stream is oxidized
to sulfur trioxide, and the slip stream is merged back into the main flue
gas flow. While of interest, this approach has major drawbacks when
implemented, for example: system thermal efficiency is reduced because
less heat is recovered; there is typically insufficient mixing of the slip
stream with the main flow at the point where they rejoin, due to an
insufficient pressure differential; and the like.
Moreover, the '516 patent does not disclose any approach which permits
control of the amount of sulfur trioxide produced, responsive to
variations in the sulfur content of the fuel and changes in other
operating parameters. A patent to a related approach, U.S. Pat. No.
3,581,463, suggests using a fan to draw a portion of the hot gas flow into
the slip stream, but gives no further details as to how the amount of
sulfur trioxide can be controlled. One can imagine that valving could be
added to the slip stream to control its total flow, but such valves are
complex, expensive, and difficult to build.
U.S. Pat. No. 5,320,052, which is assigned to the same assignee as is this
invention, provides an improvement over the approaches discussed above and
includes a catalytic converter support adapted to be disposed across at
least a portion of the cross-section of the main duct, and a catalyst for
the oxidation of sulfur dioxide to sulfur trioxide is supported by the
catalyst support. This system further includes a mechanical adjustment
means for selectively adjusting the amount of surface area of the catalyst
which is exposed to the flow of flue gas in the main duct. While it is
believed that the '052 system is an advance over the prior art discussed
hereinabove, several problems and/or deficiencies may exist, for example:
structural modifications to the duct, which are required in a retrofit
and/or new installed FGC system of this sort, is expensive and may be
difficult to achieve in many instances; mechanical complexity, with a
resultant potential for breakdown; the area required of the catalyst, and
the supporting structure at the face is relatively substantial and the may
result in a significant back pressure being created, which in turn may
result in a decrease in power plant efficiency; depending upon the various
adjustments of conversion required, the catalyst will have a tendency to
uneven wear; and the like.
In addition to the above approaches, a currently pending patent
application, which is assigned to the same assignee as is this invention,
provides for s somewhat different approach. This application includes
catalyst positioned within the main flue gas duct, and incorporates
heating and/or cooling means for selectively varying the surface
temperature of the catalyst, to take advantage of the phenomena that,
within a predetermined temperature range, conversion efficiency of the
catalyst will vary. While this arrangement has certain advantages of the
mechanical systems discussed (i.e. less mechanical complexity, simpler
retrofit, no moving parts, potential of decreased pressure loss, and the
like), certain disadvantages are readily apparent (i.e. energy cost,
capital expense, the cost of the surface temperature heating and cooling
means, the replacement of components of a temperature responsive in-duct
catalyst assembly may be more costly than replacing wear components of a
mechanical system, and others).
There is therefore a need for an improved approach to sulfur trioxide
conditioning of flue gas streams. The present invention fulfills this
need, and further provides related advantages.
SUMMARY OF THE INVENTION
The present invention provides an apparatus and method for sulfur trioxide
conditioning of flue gas streams produced by fossil fuel power plants.
This approach permits a selectively controllable amount of sulfur trioxide
to be created and added to the flue gas stream. The apparatus used to
accomplish the sulfur trioxide addition is simple and rugged, and readily
controlled to precisely vary the sulfur trioxide addition. There is no
sulfur burning apparatus or supply of sulfur required. No slip stream is
taken from the flue gas stream, and no associated variable-speed fan or
valving is used to achieve controllability. No additional sulfur dioxide
is added to the flue gas stream with the sulfur trioxide addition. There
is no difficulty in mixing the sulfur trioxide into the flue gas stream.
No overly cumbersome or difficult to maintain equipment is required.
Back-pressure caused by the system of the present invention, is reduced
from some prior systems heretofore. Only a relatively minor modification
to the duct work is required for the present invention. The conversion
efficiency of the catalyst is, under certain circumstances, increased. The
uniformity of catalyst life in a given system may be increased. Capital
and running costs may be relatively lower, and the range of conversion may
be higher. Catalyst life may be longer.
In accordance with the invention, a sulfur trioxide conditioning system is
provided for use in a fossil fuel-burning facility having a main duct for
transporting sulfur dioxide-containing flue gas from a boiler, through a
heat recovery apparatus, and to particulate removal equipment, such as an
electrostatic precipitator, for subsequent discharge through a stack. The
sulfur trioxide conditioning system includes catalytic converter means for
converting a portion of the sulfur dioxide in the flue gas to sulfur
trioxide. The catalytic converter system includes a support adapted to be
disposed across at least a portion of the cross section of the main duct,
and transversely spaced sections of catalyst carried by the support. The
conditioning system further includes an adjustment assembly which is
selectively operative to vary the quantity of flue gas flow passing
through the catalyst sections. Varying the quantity of flue gas passing by
the catalyst, will in turn vary the amount of sulfur trioxide produced by
catalytic conversion.
The conditioning system of the present invention includes flue gas flow
resistance sections carried by the support and disposed intermediate the
catalyst sections. The purpose of these sections is assure that a selected
amount of flue gas flow will in fact pass through the catalyst sections.
To insure proper flow, the pressure loss coefficient, as to flue gas
passing through the sections, is within a selected range.
The present invention provides an advance in the art of flue gas
conditioning. Other features and advantages of the present invention will
be apparent from the following more detailed description of the preferred
embodiments, taken in conjunction with the accompanying drawings, which
illustrate, by way of example, the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic depiction of a fossil fuel power plant incorporating
the principles of the present invention; and
FIG. 2 is a perspective view of one embodiment of a sulfur trioxide
assembly of the present invention, which is illustrated as being disposed
within a flue gas duct of a fossil fuel power plant.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 schematically illustrates a fossil fuel power plant 20 utilizing the
apparatus and method of the present invention therewithin. Briefly, the
power plant 20 has a combustor/boiler 22, which is supplied preheated air
through conduit 24, and fuel through fuel inlet 26. The fuel is corn
busted with the air, producing a flue gas flow 28. The flue gas flow 28
contains particulate matter (sometimes referred to as flyash), as well as
other combustion products. The flue gas flow 28 heats water flowing in
boiler tubes 30 and converts it to steam 32. The steam 32 is supplied to a
turbine/generator 34 which produces electrical power.
Flue gas flows through the primary flue gas duel 36 and thence through a
sulfur trioxide conditioning system 38 of the present invention. The flue
gas flow 28 then passes through a heat recovery apparatus 40, wherein heat
is transferred from the flue gas flow 28 to an incoming air flow 42 to
provide preheated air flow through conduit 24. After leaving the heat
recovery apparatus 40, the gas flow 28 enters an electrostatic
precipitator 44, in which a large fraction of the particulate matter is
removed by the application of electrostatic fields to the flue gas. The
flue gas flows, with most particulate removed, through an exhaust stack
46.
This discussion of the power plant 20 is intended to be highly schematic in
nature and to provide the information necessary to understand, practice,
and enable the present invention. In an operating power plant there are
typically many other systems, as well as alternative systems, that are not
shown here. The present invention is compatible with such other systems
and may be used with them.
The flue gas entering the electrostatic precipitator 44 must have enough
sulfur trioxide to react with water vapor in the flue gas to produce a
sufficient amount of sulfuric acid, which is deposited upon the surfaces
of the particulate. The sulfuric acid imparts electrical conductivity to
the particulate in the flue gas, which, as is well known, is necessary for
proper and efficient use of electrostatic precipitators 44. Sufficient
sulfur trioxide must be present to form the necessary sulfuric acid.
In the present invention, sulfur trioxide is produced in the sulfur
trioxide system 38 of the present invention, by the catalytic oxidation of
sulfur dioxide in the flue gas flow 28 to sulfur trioxide. Depending upon
the catalyst used, as well as the flue gas temperature, the catalytic
oxidation may be accomplished at a temperature above 400.degree. F., and
within a range of 400.degree. to 1400.degree. F. From present
considerations, it is believed a minimum temperature of the flue gas flow
28, as it passes by system 38, may be 500.degree. F., and in certain
situations, no less than 650.degree. F. The most appropriate temperature
range, coupled with considerations of power plant efficiency and design,
dictates that the primary portion of flue gas conditioning system 38 be
positioned within duct 36, intermediate the discharge from boiler 32, and
the heat recovery apparatus 40.
Referring now to FIG. 2, the sulfur trioxide conditioning system is
illustrated as comprising: a system support 50, which is adapted to be
suitably positioned in duct 36, for example by welding, bolting, or the
like; a plurality of catalytic converter sections 52, which are elongated
in a first transverse direction 60, with respect to the direction of the
flue gas flow 28 therethrough, are carried by the system support 50, and
are spaced with respect to each other, in a second transverse direction
62, with respect to the flue gas flow 28; elongated flow resistance
sections 54 which are also carried by the system support 50, are elongated
in the first transverse direction 60, are spaced from each other in the
second transverse direction 62, and are positioned intermediate adjacent
ones of said converter sections 52; and an adjustment assembly 58 which,
as will be described hereinafter in detail, is selectively operable to
vary the quantity of flue gas flow 28 through the catalytic converter
sections 52, while simultaneously adjusting such flow through the
resistance sections in an opposite direction. In other words, when, for
example, the adjustments assembly 58 is operative to decrease the amount
of flue gas flow through the catalytic converter sections 52, it will
simultaneously cause the amount of flue gas flow through the resistance
sections 54 to increase.
As illustrated, the sections 52 and 54 occupy substantially the entire
transverse cross section of the open area of duct 36 and, as shown, are
divided from each other by positioning and retaining walls 64, which are
carried by the system support 38. Sections 52 are comprised of catalyzed
portions 66 which are operative to promote a chemical reaction to convert
a portion of the SO.sub.2 in the flue gas stream to SO.sub.3. Any suitable
catalyst may be used for the oxidation of sulfur dioxide to sulfur
trioxide (i.e. vanadium oxide, alkali metal pyrosulfates,and alkali metal
oxides); however, for purposes of reducing the amount of catalyst
required, and hence the potential back pressure and energy requirements, a
precious metal catalyst is preferred. Any suitable configuration of
catalyst may be used, for example a honey-comb configuration or, if
preferred and conditions permit, a plate type catalyst, suitably
configured in an undulated type arrangement, to define flow passages
between adjacent plates, is acceptable.
The adjustment assembly 58 comprises: a plurality of shield assemblies 70
which are elongated in the first direction 60 and which are transversely
spaced from each other in the second direction 62; and a movement assembly
72, which communicates with shield assemblies 70 for the selective
movement thereof. Shield assemblies 70 are of any suitable configuration
and, as illustrated, the respective upstream faces 74 thereof, present a
rounded aerodynamically shaped configuration to the flue gas flow 28 to,
among other advantages, assist in the flow distribution, decrease back
pressure, and decrease wear, which latter advantage is particularly
important to increase the longevity of the very expensive converter
sections 52. It is to be noted at this point that any number of
aerodynamic shapes can be substituted for the illustrated configuration of
laces 74, by one skilled in the art.
Movement assembly 72 may be of any suitable construction and, as shown
comprise: a plurality of rod members 76 which are elongated in the second
transverse direction 62, are in sliding communication with the system
support 50 and in rigid communication with the shield assemblies 70, and
are transversely spaced with respect to each other in the first direction;
a tie assembly 78 fixedly carried by the rod members adjacent one axial
end thereof; and a worm gear 80 assembly carried by the system support 50,
and in operative communication with the tie assembly 78. With an
arrangement such as described, a suitable means of rotation, for example
an electric or hydraulic motor, not shown, is suitably energized to impart
a rotating force to turn the worm gear 80, which in turn causes the tie
assembly 78 to move in the second transverse direction 62. Inasmuch as the
rod members 76 are in rigid communication with the tie assembly 78, and
the shield assemblies 70, the movement of the tie assembly 78 causes the
shield assemblies 70 to move a corresponding amount. As illustrated, a
plurality of slide and guideways 84 which are carried by system support 50
and extend longitudinally with respect to the second transverse direction
62, are transversely spaced from each other in the first transverse
direction 60, and are included to provide a slide support and guideway for
the transverse movement of the shield assemblies 70.
As illustrated, upstream and downstream rod members 74 are included. The
reason for the upstream rod members is apparent from the drawings. As to
the downstream rod members, the invention herein anticipates suitable
aerodynamically configured shield assemblies (not shown) also being
positioned at the downstream exit side of the sections 52 and 54, to
assist in the transition of the flue gas flow 28 from such sections, and
alleviate the potential for additional back pressure being created at the
exit sides of sections 52 and 54. In this regard it is to be noted that
the latter mentioned sections will underlay and be in registry with
respect to the illustrated overlying sections 52 and 54, and will move in
tandem therewith.
Shield assemblies 70 are dimensioned and operative to cover and uncover all
or a portion of the upstream surface of the converter sections 52, in
response to sliding forces imparted thereto by the selective operation of
movement assembly 72. Thus as more of the shield assemblies 70 are
uncovered, a greater quantity of flue gas flow 28 passes through the
catalytic sections 52 (and less through the flow resistance sections 54)
and, as such, the quantity of sulfur trioxide created is increased, and
conversely when less of such upstream surface is covered. The preferred
embodiment will have the transverse dimension, when viewed in the second
direction 62, of the side of the shield assemblies 70 adjacent to the
sections 52 & 54, generally equal to the transverse dimension, again when
viewed in the second direction 62, of the of the sections 52. This
arrangement will provide maximum balance, economics and efficient use of
the catalyzed portions 52. Other matters to be considered with respect to
the shield assemblies 70, include: the shield assemblies can caused to
selectively move twice the transverse dimension thereof, when viewed in
the second direction 52, thus incorporating an arrangement that selected
sides of the catalyzed portions 66 can be alternately exposed to the flue
gas flow 28 (hence resulting in potentially more uniform wear of the
catalyzed portions); and, if desired, suitable soot blowing assemblies
(not shown) can be positioned within the shield assemblies and be
selectively energized to assist in the removal of accumulated particulate
for the converter sections 52 and/or at least portions of the resistance
sections 54.
The face area of the converter sections 52, with respect to the inside face
area of the system support 50 will vary, dependent upon a number of
matters (i.e. the temperature of the flue gas flow 28, the type of
catalyst used, the velocity of the flue gas flow 28, the type of fuel
being burned, the amount of sulfur trioxide to be generated, pressure loss
coefficient considerations, and the like); however, it is anticipated that
such first mentioned face area shall be within the range of 10% to 50% of
the face area of such system support 50, and preferably less than 30% of
the last mentioned face area. On related matters, it is anticipated that,
in many instances, the transverse spacing between the converter sections
52 will be substantially equal, as will the transverse spacing between the
resistance sections 54 (i.e. albeit in such situations it is not
necessarily accurate that the transverse spacing between the converter
sections 52 will equal the transverse spacing between the resistance
sections 54). On the other hand, conditions (i.e. uneven flue gas flow 28,
turbulence considerations and the like) may dictate that, rather than
substantially equal spacing, a preselected spacing be selected based on
mathematical, modeling and experience considerations.
An important consideration of the invention herein resides in a recognition
that, to insure an efficient use of catalyst, and appropriate conversion
efficiency for a given set of circumstances, comparative considerations of
relative flow resistance of the pressure loss coefficients as to the flue
gas flow 28 through the sections 52 & 54, respectively, must be made. In
this regard, the Applicant has determined that the ratio of the pressure
loss coefficient, as to the flue gas stream 28 passing through the
resistance sections 54, with respect to the pressure loss coefficient, as
to the flue gas stream passing through the converter sections 52, should
be in the range of 0.2 to 2, and preferable 0.5 to 1.0. This coefficient
can be established in any suitable manner, for example by dividing the
static pressure drop by the dynamic or velocity head, all in a manner as
is well known to one skilled in the art. These coefficient will be
determined to some extent by the relative make up of the sections 52 and
54. In this regard, efficiency considerations will dictate relatively
closer spacing of the honey comb channels or plate spacing of the
catalyzed portions, when compared to the like spacing of the "fill" for
the flow resistance sections 54 (Note: the flow resistance sections can be
"filled" with any suitable configurations materials, for example grating
or relatively large undulated plates.) It is to be noted that, because of
the relative coefficients, changes in the relative exposure of specific
face areas of sections 52 & 54 to the flue gas flow 28, does not
necessarily create a straight line variation of the quantity of flow
through such sections.
When considering the above discussion, as well as any number of operating
parameters, certain additional preferred or practical criteria has been
developed, for example: the range of desired conversion of sulfur dioxide
to sulfur trioxide in the flue gas flow, should be in the range of
substantially 0% to 20%, and often in the range of substantially 0% to 3%,
in response to the selective operation of adjustment assembly 58; the
sulfur trioxide conditioning system, when positioned within the duct 36,
should result in an additional pressure loss of no more than 5 inches
water gage, and, preferably, no more than 2 inches water gage.
The present invention is particularly suitable for direct automatic
adjustment of assembly 58. In this regard a number of signals can feed
back to a suitable microprocessor controller (not shown) to selectively
control the degree of movement of the shield assemblies 70, in response to
operational parameters. Examples of such signals can include, by way of
example, but not limitation: sensor 82a, the power consumed by the
electrostatic precipitator 44 (a measure of the amount of particulate
being collected); sensor 82b, the boiler load; sensor 82c, the chemical
composition of the flue gas, including the sulfur dioxide and sulfur
trioxide contents, sensor 82d, the exhaust gas temperature; sensor 82e,
the electrical properties of the fly ash particulate, such as its
resistivity; and/or sensor 82f, the stack gas opacity. The construction of
each of these sensors are individually well known in the art.
although a particular embodiment of the invention has been described in
detail for purposes of illustration, various modifications may be made
without departing from the spirit and scope of the invention; for example,
one or more doors can be positioned in the system support 50 to assist in
the removal and insertion of the catalyst; the configuration of shield
assemblies can be modified to suit conditions: other forms of driving
arrangements may be used for moving the shield assemblies 70; and the
like. Accordingly, the invention is not to be limited, except as by the
appended claims.
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